Plasma waves, observed above the dayside Venus ionosphere by the
Pioneer Venus Orbiter (PVO) plasma wave instrument, have been
attributed to whistler mode waves, lower hybrid waves, or ion acoustic
waves. In order to clarify the nature of the waves, we have performed
both case study and statistical analyses of the plasma wave and
magnetic field data. We find that the plasma wave data are well ordered
by altitude with respect to the location where the PVO Langmuir probe,
or orbiter electron temperature probe (OETP), measures a density of 100
cm, known as the OETP ionopause.
The dominant signature in the wave data appears to be a change in the
instrument noise level because of changes in the plasma Debye length.
However, there is a burst of wave activity near the OETP ionopause.
Also, we find that there is a rotation in the magnetic field at or near
this location. By casting the magnetic field data into a coordinate
system ordered by the presumed magnetosheath flow, we find that the
rotation of the field tends to orient the field in a more flow-aligned
direction at lower altitudes. We attribute this to mass loading at
lower altitudes. We further suggest that the field-aligned current
associated with the field rotation corresponds to a shear Alfvén
wave standing in the magnetosheath flow. The field-aligned currents are
present because of boundary conditions imposed on the flow, and it is
not clear if the waves are actually associated with the field-aligned
currents or are simply coincidental. Since the waves are observed at
the OETP ionopause, further progress in understanding these waves will
be made though determining what underlying plasma structure, if any, is
related to the OETP ionopause, which is defined by a specific
instrument threshold. Nevertheless, our study confirms that the wave
activity, field-aligned currents, and OETP ionopause all occur within
the plasma mantle above the ionosphere. As such, the plasma waves are
not an energy source for the dayside ionosphere.